For many years sewing machines have been used to join sheets of fabric together by basically tying the sheets together with thread. Recently, garment manufacturers have started joining sheets of fabric together by welding overlapping edges using heat and pressure. When heat is applied, the overlapping edge portions of the sheets melt and fuse together to form a seam. This method works reasonably well with certain thermoplastic materials, but less well with many other materials, such as certain woven fabrics that can become brittle after heating and thereafter easily rip.
An alternative to creating seams using welding techniques involves joining fabric together using an adhesive. One such known technique utilizes a strip of adhesive positioned between overlapping edges of sheets of fabric. Pressure and heat are applied to the overlapping fabric and the adhesive to melt the adhesive and form an adhesive bond that joins the overlapping fabric sheets. Known adhesively bonded seams exhibit several limitations, depending upon the manner of joining the fabric and the type of fabric used.
One example of a known adhesively bonded seam is illustrated in FIG. 1. With reference to FIG. 1, the fabric sheets are joined “face-to-face” similarly to a standard sewn seam, resulting in a glued seam that is subject to separation and failure when opposing tensile forces F are applied to the fabric sheets. In this example, the forces F are shown at an angle of approximately 900 to the plane of the seam. Such seams are subject to failure due to the fact that the forces F are applied to the seam at a substantial angle relative to the plane of the seam. When the angle is significant, as in the example of FIG. 1, the tensile forces F are primarily acting on the outer edge of the seam, where the two sheets of fabric meet at the edge of the adhesive (and from where the sheets diverge). The concentration of stress along the edge of this seam may cause the seam to peel apart and fail.
Another known type of adhesively bonded seam, illustrated in FIG. 2, is typically formed by overlapping the outside or facing surface of a first sheet of fabric to the inside or backing surface of a second sheet of fabric and joining the overlapping surfaces with adhesive. In this instance, the resulting adhesively bonded lap seam is typically stronger than the seam of FIG. 1 because the tensile forces F are applied approximately parallel to the seam of FIG. 2. However, the seam of FIG. 2 results in a raw edge of material being exposed on the outside of the garment, and the raw edge may easily fray or unravel in normal use. The seam structure of FIG. 2 may also not work well with many modern fabrics of the kind having a laminate structure comprising an outer facing fabric, having a high tensile strength, and an inner backing fabric, having a relatively low tensile strength. When tensile forces F are applied to sheets of laminate fabric of this kind that are adhesively joined as depicted in FIG. 2, the inner backing fabric may be prone to de-laminate or otherwise separate from the outer facing fabric, ruining the garment.
Thus, the present inventors have recognized a need for improved adhesively bonded seams and methods of forming such seams.